1. Field of Invention
The present invention relates to a transistor having a start-up control element. More particularly, the present invention relates to a metal-oxide-semiconductor field-effect transistor (MOSFET) having a start-up control element as a power switch.
2. Description of Related Art
A start-up circuit is often required when a power supply circuit supplies voltage to a power circuit within an integrated circuit (IC). The start-up circuit provides a starting bias voltage to the power circuit until the power circuit is able to function normally. Afterwards, the start-up circuit is expected to be idle and consume no power, if ideally. FIG. 1 is a diagram illustrating relationships among a start-up circuit 10, a power supply 100 and a power circuit 200. During initialization stage, the power circuit 200 has not been fed with power. Therefore, it is necessary to provide a start-up circuit 10 to charge the capacitor C until the voltage at the node Vbias reaches a predetermined value that is able to turn ON the power circuit 200. After the power circuit 200 is turned ON, it may operate without any aid from the start-up circuit 10. For example, the power circuit 200 may obtain power from the power supply 100 via some other approach (other than the start-up circuit 10) and transfer the power into a low DC voltage Vdd required by the IC. The details are not described here for that they are well known to those skilled in the art.
FIG. 2 is a diagram illustrating a prior art start-up circuit. Since the start-up circuit 10 is expected to consume as little current as possible, the simplest approach is to provide a resistor 20 of high resistance. The resistor 20 transfers the voltage from the power supply 100 to a low current, charging the capacitor C until the node Vbias reaches a predetermined voltage value. The voltage at the node Vbias, for example, may be provided to drive a pulse width modulation (PWM) circuit 12 in the power circuit 200, and the power circuit operates under the control of the PWM circuit 12. The details of the PWM circuit and how it controls the power circuit 200 are not described here for that they are well known to those skilled in the art.
According to the prior art illustrated in FIG. 2, the resistance of the resistor 20 must be quite large to limit the current, because the voltage provided by the power supply 100 is quite high. Accordingly, the area of the resistor 20 inevitably becomes very large, and a huge amount of heat is generated. Moreover, such start-up circuit cannot be turned off; the serious problems of power consumption and heat generation go on even after the power circuit has been started up.
Another start-up circuit is disclosed in U.S. Pat. No. 5,285,369, “Switched Mode Power Supply Integrated Circuit with Start up Self Biasing”. The disclosed circuit is very complicated, and a simplified form thereof is illustrated in FIG. 3. This prior art utilizes the characteristics of the parasitic junction transistor inherently existing with a metal-oxide-semiconductor field-effect transistor (MOSFET) As shown in the figure, the MOSFET 84 may be taken as a combination of a junction field-effect transistor (JFET) 86 and a MOSFET 88. The JFET 86 is a depletion mode transistor, inherently capable of limiting current, and it is normally in an ON state as its gate is electrically connected to ground. The node between the JFET 86 and the MOSFET 88 provides current for starting up a control circuit 14. The control circuit 14 provides two functions: on the one hand, the control circuit 14 charges the capacitor C; on the other hand, when the voltage at the node Vbias reaches a predetermined value, the control circuit 14 generates a control signal to switch off the MOSFET 88 and turn off the start-up circuit formed by the MOSFET 84 and the control circuit 14.
Though the conventional start-up circuit illustrated in FIG. 3 can be automatically turned off and the heat generated by the circuit is much less than that in FIG. 2, the structure of the control circuit 14 is too complicated (as may be understood by referring to the details thereof), which is undesired.
Therefore, another circuit structure is disclosed in U.S. Pat. No. 5,477,175 “Off-Line Bootstrap Start up Circuit”, which is simpler than the circuit in FIG. 3. As illustrated in FIG. 4, the circuit disclosed in U.S. Pat. No. 5,477,175 obtains current from the node between the JFET 101 and the MOSFET 102, and transfers the current to voltage by a resistor 103, which is supplied to the gate of the MOSFET 102 to turn ON the MOSFET 102. After the power circuit 200 is started, the transistor 109 can be switched off by controlling the node 113. That is, the current flowing through the resistor 103 can be turned off.
Though the complexity of the circuit illustrated in FIG. 4 is reduced as compared to the circuit disclosed in U.S. Pat. No. 5,285,369, it is still not satisfactory. Namely, the utilization of the resistor 103 still results in heat dissipation problem. It is true that the resistor 103 needs not be too large because it is only required to provide a voltage sufficient to turn ON the MOSFET 102, and hence the heat dissipation problem is less severe than the circuit illustrated in FIG. 2. However, it is still not good enough.
In view of the foregoing, it is desired to provide an advanced start-up circuit which has a simple circuit structure but does not include any resistor, to avoid the drawbacks in the prior art.